The wireless portable Internet is a next-generation communication system that supports mobility in LAN (Local Area Network) data communication systems using a fixed access point such as the existing wireless LAN system. A variety of standards for the wireless portable Internet have been developed, and international standardization of portable Internet is now in progress actively on the IEEE 802.16 standardization group.
FIG. 1 is a schematic of a wireless portable Internet system under development in the IEEE 802.16 standardization group.
The wireless portable Internet system basically includes a subscriber station (SS) 10, base stations 20 and 21 performing wireless communication with the subscriber station 10, routers 30 and 31 being connected to the base stations through a gateway, and an Internet network.
The wireless portable Internet system of FIG. 1 secures the mobility of the subscriber station 10 even when the subscriber station 10 is moving from a cell managed by the base station 20 to a cell imaged by the base station 21, thereby providing a seamless data communication service. Therefore, the wireless portable Internet system supports a handover of the subscriber station 10 as in the mobile communication service, and performs dynamic IP address allocation according to the movement of the subscriber station 10.
FIG. 2 is a flow chart showing a connection establishment process in the wireless portable Internet system.
As the subscriber station 10 enters the cell of the base station 20, in step S1, the base station 20 acquires downlink synchronization with the subscriber station 10, in step S2. With the downlink synchronization a the subscriber station 10 acquires an uplink parameter, in step S3.
Subsequently, ranging between the subscriber station 10 and the base station 20 is achieved, in step S4. The ranging procedure, which is the adjustment of timing, power, and frequency offset between the subscriber station and the base station, includes an initial ranging step and a subsequent periodic ranging step.
After the completion of the ranging procedure, negotiation of the basic (service) capabilities for connection establishment between the subscriber station 10 and the base station 20 is performed, in step S5. Once the negotiation of the basic capabilities is completed, the base station 20 authenticates the subscriber station 10 using an equipment identifier such as the subscriber station's MAC (Medium Access Control) address, and a certificate such as X.509 Digital Certificate issued by the manufacturers in step S6.
When the subscriber station 10 is authenticated and authorized to use the wireless portable Internet service, the equipment address and capabilities for services of the subscriber station 10 is registered, in step S7. Then, the basic station 20 provides an IP address received from an IP address management system such as a DHCP (Dynamic Host Configuration Protocol) server to the subscriber station 10 to establish the IP connection, in step S8. Upon receiving the IP address, the subscriber station 10 performs connection establishment for data transfer, in step S9.
The bandwidth request method of the subscriber station from the base station is divided into a non-contention-based bandwidth request method, and a contention-based bandwidth request method.
The non-contention-based request method includes a unicast polling method similar to the existing messaging-based bandwidth request method, and a piggyback method.
Namely, the unicast polling method or the piggyback method can be used efficiently when a bandwidth available to the corresponding subscriber station can be allocated to the uplink. Otherwise, a contention-based bandwidth request method is required instead of those methods.
Hence, the portable Internet system under development in the IEEE 802.16 standardization group employs a CDMA-based bandwidth request method as a contention-based bandwidth request method.
In the portable Internet system suggested by the IEEE 802.16 standardization group, a CDMA-based periodic ranging request and a CDMA-based bandwidth request are transmitted on the same uplink domain. The bandwidth request is for the subscriber station to request uplink bandwidth allocation from the base station, and the periodic ranging request is to periodically adjust the system based on the channel state. Here, two ranging codes or preambles (hereinafter, codes and preambles are used in the same meaning) used for the bandwidth request and the periodic ranging request are identified by the base station using a dynamically allocated CUG (Code Usage Group) in UCD (Uplink Channel Descriptor) message transmitted to the subscriber station.
FIG. 3 shows a bandwidth request method of the portable Internet system according to prior art.
Referring to FIG. 3, after a ranging timer times out, a mobile station SS transmits a periodic ranging code to a base station BS, in step S41. The base station BS transmits a ranging response message RNG-RSP specifying state control information to the subscriber station SS, in step S42. Upon receiving the ranging response message for the periodic ranging from the base station BS, the subscriber station SS adjusts timing, power, frequency offsets based on the state control information received from the base station, in step S43. The periodic ranging procedure is iterated in units of a predetermined interval T1 (hereinafter, referred to as “periodic ranging interval”).
When the subscriber station SS intends to transmit uplink data, in step S44, it transmits a bandwidth request code BW Req Code to the base station BS, in step S45. Upon receiving the bandwidth request code, the base station BS allocates a resource for transmission of a bandwidth request message to the subscriber station SS, in step S46. The base station BS also allocates information (e.g., frame number, slot number, sub-channel number, code number, etc.) for discrimination of the subscriber station transmitting the code using CDMA_Allocation_IE of the uplink map UL-MAP, because it cannot discriminate the subscriber station transmitting the bandwidth request code (or preamble) from the identifier of the subscriber station SS. Upon receiving the information, the subscriber station SS transmits a desired bandwidth value, i.e., a bandwidth request header BW Req Header and/or uplink data (UL Data) in step S47
According to the conventional method of FIG. 3, the control of timing, power, and frequency based on the channel state is achieved only by periodic ranging, without a separate control operation during a bandwidth request. As a result, data are transmitted without control information based on the channel state using a bandwidth allocated after transmission of a bandwidth request code. In the fixed wireless environment in which the channel state does not vary so frequently, it has only to control the subscriber station based on the channel state using periodic ranging repeated in units of a predetermined interval T1, as shown in FIG. 3. In the mobile environment that has the channel state changing very frequently relative to the wireless environment, however, a proper adaptive modulation and coding (AMC) level based on the channel state cannot be applied in using the method of FIG. 3, and accordingly, efficient transmission becomes more difficult. In addition, interference occurs to the data of subscribers using adjacent sub-channels when the timing as a state control information is not matched.